Air-entraining agents entrain or retain air in the fresh concrete by promoting the formation of stable and dispersed microscopic air voids. Air-entraining agents contain surface-active agents which concentrate at the air/water interface and lower the surface tension so that air voids can form and stabilize more readily. Surface-active agents are molecules, which at one end have chemical groups that tend to dissolve in water (hydrophilic groups) and adhere to wetted particles of fly ash and cement. At the other end, the surface-active agents have chemical groups that are repelled by water (hydrophobic groups). AEA's surface-active molecules tend to align at the air/water interface with their hydrophilic groups in the water and the hydrophobic groups in air. The hydrophobic groups have the affinity to also adhere to activated carbon or unburned carbon introduced into the concrete by the fly ash. Hydrophobic groups that adsorb on carbon surfaces are not available to entrain air voids. The loss of an AEA's ability to entrain an adequate amount of air is detrimental to the durability of concrete.
The volume of air voids required to provide optimum freeze-thaw protection in concrete is practically maintained in the 4%-8% range or more desirably 5%-7% by volume of concrete. The presence of activated carbon, which adsorbs AEAs, from fly ash in concrete, causes less than an optimal air void content to be obtained in concrete; thus, resulting concrete is more susceptible to damage from frost formation in concrete pores.
To mitigate the impact of activated carbon, the ash can be treated with a sacrificial agent to passivate the carbon adsorption capacity and prevent it from adsorbing excessive amounts of AEAs when used in concrete. In order to determine the accurate dosage for addition of the sacrificial agent, the adsorption capacity of carbon containing fly ash must be accurately determined. The level of carbon in ash and its adsorption capacity vary depending on power plant operating conditions. Electric power unit generation load, coal ash content, activated carbon injection rate, activated carbon surface area, operating conditions of burner, boiler, and air pollution control systems, etc., result in varying levels of carbon and variability in ash adsorption capacity. In order to determine the appropriate dosage of the sacrificial agent, the ash adsorption capacity must be periodically determined. The manual foam index test is typically used to determine the fly ash affinity to adsorb AEAs. The foam index test is subjective and cannot be automated. Methylene blue or acid blue adsorption can be used to determine the adsorption capacity of fly ash. These manual test procedures have been used in the lab and for field testing to periodically determine ash adsorption capacity.